Engine systems may utilize one or more turbochargers to compress ambient intake air entering the engine to provide increased power output. Turbocharged engine systems may also be configured with one or more high-pressure and/or low-pressure exhaust gas recirculation (EGR) systems that recirculate at least a portion of the exhaust gas to the engine intake. In an engine system utilizing two turbochargers operating in parallel, separate EGR systems may be associated with each turbocharger. Utilizing separate EGR systems necessitates a footprint and packaging space large enough to accommodate components for both EGR systems.
With respect to a typical low-pressure EGR system in a turbocharged engine, a potential issue can arise when certain environmental conditions exist, such as low ambient air temperature and/or high humidity conditions. Under these conditions, water vapor condensation may form on internal surfaces of engine components, such as the charge air and EGR air coolers, when those surfaces are cooler than the saturation temperature (dew point) of the exhaust gas and/or exhaust gas/charge air mixture. Under certain operating conditions such as hard accelerations, the condensation may be dislodged, for example, from the EGR air cooler and may travel into a turbocharger compressor, potentially damaging compressor components. The condensation may also continue into the combustion chambers of the engine causing performance issues, such as torque and engine speed losses, engine misfires and incomplete fuel burn.
A low-pressure EGR system also increases the intake air temperature that is experienced by the compressor. If not controlled properly, the compressor outlet temperature may rise to levels that can degrade compressor components. A low-pressure EGR system may also contribute to a compressor surge condition, in which an unstable and inefficient air flow condition can reduce compressor performance and potentially damage compressor components.
The inventors herein have recognized the above issues, as well as various solutions to address them.
In one example, the above issues may be at least partly addressed by a method of operating an engine including a first turbocharger having a first compressor and a second turbocharger having a second compressor. In one embodiment, the method comprises increasing an EGR differential between the first compressor and the second compressor under a condensation condition, and decreasing the EGR differential between the first and second compressors under a surge condition.
For example, when a condensation condition exists, such as when an air intake temperature is below an intake temperature threshold, the EGR differential between the first compressor and the second compressor may be increased by reducing exhaust gas flow to the second compressor, and correspondingly increasing exhaust gas flow to the first compressor. In this manner, the amount of heated exhaust gas combined with intake air is increased, thereby increasing the air intake temperature at the compressor inlet and reducing the likelihood of condensation accumulation.
When a surge condition exists, such as when an estimated manifold pressure exceeds a pressure threshold and an estimated air flow downstream from the first compressor is below an air flow threshold, the EGR differential between first compressor and the second compressor may be decreased by increasing exhaust gas flow to the second compressor, and correspondingly decreasing exhaust gas flow to the first compressor. In this manner, the amount of exhaust gas combined with intake air is decreased, thereby decreasing the air flow into the first compressor inlet and reducing the likelihood of compressor damage due to a surge condition.
In another example, when an estimated compressor outlet temperature exceeds an outlet temperature threshold associated with potential damage to compressor components, the EGR differential between the first compressor and the second compressor may be decreased by increasing exhaust gas flow to the second compressor, and correspondingly decreasing exhaust gas flow to the first compressor. In this manner, the amount of heated exhaust gas combined with intake air is decreased, thereby decreasing the air intake temperature at the first compressor inlet and correspondingly decreasing the compressor outlet temperature, and reducing the likelihood of compressor degradation due to high temperatures.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.